A prior art power generator using a fuel cell is described below with reference to FIGS. 3 and 4. FIG. 3 is a diagram showing the configuration of a prior art fuel cell power generator. FIG. 4 is a diagram showing the configuration of a control circuit of the prior art fuel cell power generator. In a fuel cell 1 shown in FIG. 3, an air electrode 2 and a fuel electrode 3 are arranged such as to sandwich a polymer electrolyte membrane 4, and the upstream side of the air electrode 2 is connected to a blower 5 for supplying air. A hydrogen generator (fuel gas generator) 6 is supplied with a power generation fuel X serving as a raw material such as natural gas or methanol and water Y necessary for a steam reforming reaction, and the obtained fuel gas G is supplied through a switching valve 7 to the fuel electrode 3, and thereby flows downstream through a predetermined passage contacting with the fuel electrode 3. At that time, a required amount of hydrogen in the fuel gas G is consumed in an electrode reaction, whereby the residual hydrogen and the like are supplied as off-gas OG to a burner 8. When the fuel gas G is not supplied to the fuel electrode 3, the fuel gas G is supplied through the switching valve 7 to the burner 8.
In general, when the fuel cell power generator is in the off state, the passages for the fuel gas G and the off-gas OG in the fuel gas generator 6, the fuel electrode 3, and the like are filled with an inert gas such as nitrogen. Further, even when the fuel cell power generator is started up, until the temperature of the fuel gas generator 6 becomes stable, the concentration of carbon monoxide in the fuel gas G is high. The carbon monoxide degrades an electrode catalyst in the polymer electrolyte membrane 4 of the fuel cell 1 and, accordingly, the fuel gas G having a high carbon monoxide concentration is not allowed to go to the fuel cell 1 and is supplied through the switching valve 7 to the burner 8 for a few tens minutes to a few hours after the start up. When the time of a few tens minutes to a few hours has been elapsed after the start up and the temperature of the fuel gas generator 6 has become stable, the fuel gas G is supplied through the switching valve 7 to the fuel electrode 3 for starting power generation of the fuel cell 1.
The fuel gas G supplied to the burner 8 or the off-gas OG combusts with air supplied from a fan 9 to form a flame 11 in a combustion chamber 10, whereby the combusted gas heats the fuel gas generator 6. The flame 11 in the combustion chamber 10 is detected on the basis of the ion current, which is generated when a predetermined voltage is applied on the flame.
As shown in FIG. 4, a flame detector 12 is comprised of: a DC power supply 14 for applying a predetermined voltage on the electric conductor 13 and the burner 8 while using the flame 11 as a flame resister; a resistor (RA) 15 for converting a current (IRA) equivalent to the ion current (IF) flowing through the flame 11, into a voltage; a voltage detection section 16 for detecting the voltage across the resistor (RA) 15; and a control section (not shown) for controlling them, in order to measure the ion current which flows to the burner 8 via a heat resistive electric conductor 13 provided such as to contact with the flame 11.
As shown in FIG. 4, the control section (control circuit) of the flame detector 12 forms a current mirror circuit is formed by: transistors (Q1) 17 and (Q2) 18 each having the same characteristics; and resistors (R1) 19 and (R2) 20 each having the same resistance, in order to let a current (IRA) equivalent to the ion current (IF) flowing through the flame resistor (RF) flow through the flame resistor (RA) 15. Accordingly, currents (IR1) and (IR2) flowing respectively through the resistors (R1) 19 and (R2) 20 are equal to each other and a current equal to the currents (IR1) and (IR2) flows as the current (IRA) equivalent to the ion current (IF) flowing through the flame 11 to generate a voltage across the resistor (RA) 15. As such, the flame detection section 12 detects the combustion state such as ignition and misfire of the flame 11.
In such a prior art fuel cell power generator, the concentrations of hydrocarbon in the fuel gas G and the off-gas OG are extremely low because the hydrocarbon in the raw material for power generation X is converted into hydrogen by the steam reforming reaction. When the concentration of hydrocarbon is low, the ion concentration also becomes low in the flame 11, the current value flowing through the flame 11 is reduced and, hence, the voltage across the resistor (RA) 15 is reduced. That is, there has been the problem that the detection voltage in the flame detection section 12 becomes low to cause a difficulty in state recognition at the times of ignition and misfire. For example, when misfire is erroneously determined as ignition and the fuel gas is supplied continuously, under the condition that the combustion section is at a high temperature (400° C. or higher), there is a danger of explosive ignition when the concentration reaches the limit of explosion or higher. In contrast, when ignition is erroneously determined as misfire, such an unnecessary determination of misfire may cause a defect such as stoppage of the equipment operation.
Thus, in order to resolve the above-mentioned problems in the prior art, an object of the invention is to provide a fuel cell power generator capable of reliably recognizing ignition and misfire of a burner for heating a fuel gas generator and operating safely.